Such an instrument is conventionally elongate with a shank adapted to be engaged in a manual or motorized rotational drive apparatus, and a working part of which the working surface, extending from one end, provides the drilling or boring function.
As is known, a dental operation such as osteotomy, generally involves forming or enlarging a channel in the jawbone, sometimes using a an instrument rotationally driven at high speed, and it generally required of such a dental instrument to have a low coefficient of friction with the jawbone (which is promoted by the presence of lubrication, sometimes through the instrument), a high mechanical hardness conferring good wear resistance, good corrosion resistance, in particular to the acids of saliva, and good thermal conductivity to efficiently remove the heat generated when drilling a channel, while being biocompatible.
To meet such a combination of needs, it has been proposed, in particular by document U.S. Pat. No. 5,299,937, to cover at least the working surface of a dental instrument with a diamond type hard carbon coating (often referred to as DLC, standing for Diamond-Like-Carbon); more recently, the document EP-1 128 777 recommended selecting such a diamond-like-carbon coating containing between 5 at. % and 35 at. % hydrogen.
In fact, when a dental instrument is used for osteotomy, manipulated by hand or actuated by a rotational drive motor, it is useful to be able to evaluate as precisely as possible the depth to which the end of the instrument has reached in the bone. It was to meet that need that it was proposed, in particular in the aforementioned document EP-1 128 777, to form visible depth markers on the working surface of the instrument, such as annular bands surrounding the working part of the instrument that are produced so as to form a visible contrast with the rest of the surface of that working part; in practice these depth bands are formed by locally removing the whole of or part of the thickness of the diamond-like-carbon coating. In practice, these bands are equidistant; furthermore, there are instruments on which the bands are of a width substantially equal to the distance separating them.
Such a configuration is satisfactory in numerous cases but it has been found that, in certain orientations, during certain operations, the visible contrast between the depth bands becomes insufficient to enable reliable evaluation of the depth which the end of the instrument used has reached.